1. FIELD OF THE INVENTION
This invention relates to the assembly of integrated circuit devices and, in particular, to a technique of automatic wire bonding utilizing single lead clamping at individual bond sites.
2. PRIOR ART
In the construction of integrated circuit devices, that is, chips, dies, etc., typically a semiconductor chip is mounted on a base and wiring connections are established from a lead frame placed around the chip to various terminal pads located on the chip. The techniques of actually accomplishing the electrical connections vary from fully automatic to hybrids requiring actual operator manual steps. Given the extremely fine wire size and the close dimensions involved, this step is time consuming and a source of defects in product as a result of incomplete or poor bonds being established.
Another source of the difficulty in achieving a high degree of repeatability at this step in semiconductor manufacture is that generally the chip is provided with an alpha barrier having a double sided adhesive film. That is, the alpha barrier is placed over a major surface of the semiconductor device and held in place with the adhesive film on one side. Then, during assembly, a lead frame strip with multiple device sites is overlaid on the alpha barrier material and held in place by the adhesive film on the top of the alpha barrier layer. At this intermediate stage of manufacture, the integrated circuit device (such as a chip, die) has the alpha barrier on a major surface and fixed onto the alpha surface is the lead frame having a series of metallic leads stabilized by means of the adhesive film on the alpha barrier. Should there be any misalignment, the leads on the lead frame may have longer or shorter effective lengths with the attendant differences in bending when the bonding tool is placed in contact.
The next step in fabrication is the bonding of selected terminal pads on the semiconductor device followed by bonding of selected leads from the lead frame to the device so that conductive wires may be fixed between the terminal ends of the lead frame conductors. Within the prior art there are a number of systems used to bond the pads on the device and the conductive wires to the lead frame fingers. However, the bonding is done in an unstabilized mode such that the wires may not be sufficiently held in place during the bonding step, or, in an extreme situation, the bond may take place between incorrect terminals.
Within the prior art a number of devices have been proposed for clamping metallic leads over a resilient film or tape at bonding sites. The clamping can be used for a variety of techniques. For example, U.S. Pat. No. 4,437,604 discloses the use of a wire clamp which is utilized to break the wire when the bonding tool is moved from the second bond location to begin metallic bonding at a new site. The technique however, does not clamp to stabilize the lead to be bonded at a single bond site. That is, the bonding tool is moved to a first location where an ultrafine wire is bonded to a terminal on the semiconductor device utilizing a combination of ultrasonic energy and pressure applied by the bonding tool. Then, either the bonding tool holder or the workpiece holder upon which the semiconductor device is mounted is moved to cause the wire to be fed out from the capillary bonding tool to form a length of wire which will be a fine wire interconnection between the terminal pad on the semiconductor device and the conductive terminal (lead frame finger) on the lead frame.
The bonding tool is then located over an appropriate terminal on the lead frame and a second bond is made again, by the application of force and ultrasonic energy applied by the bonding tool. Then, the wire is clamped and the table is moved to break the wire and form a wire bonding tail in the ultrasonic bonding tool. There is no stabilization of the lead frame at the location where the second bond is to take place. Another scheme used as a wire tail puller after the second bond is disclosed in U.S. Pat. No. 3,806,019. In the scheme of this patent, a wire clamp is operable to grip the wire and pull it away after the wire has been cut by a bonding torch. Reference is made to U.S. Pat. No. 3,894,671 which discloses a different technique of press clamping a lead frame strip to a heater block during bonding. Similarly, U.S. Pat. No. 4,527,730 discloses a wire loop shaping guide and clamp which is used to hold the wire independent of the bonding tool. The clamp, however, does not stabilize the conductive terminals on a lead frame to be bonded at a single bond site. Other techniques for wire clamping or combinations of clamp and feed devices for use in wire bonding tools are disclosed in U.S. Pat. Nos. 4,142,714; 3,672,556; 3,863,827; 4,475,681; and 4,202,482. The systems proposed in these patents are wire feeds used in conjunction with techniques for clamping the wire during feeding. Clamping is not effectuated for purposes of stabilizing the lead frame finger during wire bonding at a single site. Another class of prior art which utilizes clamping techniques is in detector schemes to determine whether a wire is present within the clamp. Such fault detectors are disclosed in U.S. Pat. Nos. 4,213,556 and 4,485,957.
Finally, within the prior art, techniques exists utilizing modular clamping fingers to clamp groups of metallized leads over a resilient film or tape at the bonding site. Such hold down techniques used in conjunction with ultrasonic or thermosonic wire bonding techniques are disclosed in "Research Disclosure", June 1986, No. 266, as disclosure 26622.
A hallmark deficiency of all prior art systems described herein is their inability to provide an affirmative clamp mechanism which stabilizes lead terminals on a lead frame to be bonded at single bond sites. Consequently, within the prior art, complicated workholder designs are required to fix the lead frame in position to allow an automatic wire bonding tool to step across the semiconductor device and make appropriate wiring connections.